20mm? I bet you mean 40mm....
Good job on dusting and the fan. Is the green shroud staying on or off? Might want to test it both ways and see what helps. Hate to burst your bubble, but I doubt that front fan will do much good. Not with those drives in the way.
I think mine is a 40mm aswell, (the NB fan I installed)
The green duct has to stay on as it lets the exhaust fan suck all of the cool air directly through the CPU heatsink.
To be honest, If I put my hand near the top of the HDD theres plenty of air being pushed through, its a lot better than all of that dust in the way thats for sure!
Also It cools the HDDs down, as they get kinda hot.
When I get a chance Im going to attempt to put the HDDs horizontally so that even more air can get in! Im curious to see what the panty hose will look like in 20 days.
BTW with the green duct off, not much air passes through the heatsink and it gets hot.